During the fabrication of a semiconductor device, a plasma is often used to subject a semiconductor wafer (hereinafter called a wafer) to a process such as film-formation or etching. This processing is performed by introducing a processing gas into a vacuum vessel which is provided with a mounting stand for a wafer, then supplying energy such as electromagnetic energy to that processing gas to create a plasma therefrom. Various methods are known as methods of supplying this electromagnetic energy, such as electron cyclotron resonance (ECR), which makes use of the interaction between microwaves and a magnetic field, and a method known as inductive coupled plasma (ICP) in which an electric field and a magnetic field from a coil surrounding a dome-shaped vessel are applied to a processing gas.
A high-density plasma is established in a zone corresponding to the wafer, but a thin plasma is also present throughout the entire interior of the vacuum vessel. This deteriorates an O-ring that acts as a sealing material and also causes peeling of a film that has adhered to the walls of the vessel due the reactions of the processing gas. It is therefore not possible to avoid the generation of particles within the vacuum vessel. Particularly with plasma processing, the speed of movement of electrons within the plasma is greater than that of ions, so a large number of electrons adhere to surfaces such as the walls and internal members of the vessel, as well as the wafer, and are negatively charged. Thus the potential gradient in the vicinity of these surfaces destroys the neutral characteristics of the plasma in the vicinity of the surfaces, without allowing the electrons to approach, thus forming a zone called a sheath of a thickness of a few mm.
Since the particles are negatively charged, they cannot pass through the sheath zone and are pushed back towards the plasma. As seen from a certain point in time, it occurs that these particles appear to be trapped at the boundary between the plasma and the sheath zone. However, when the microwave power and wafer biasing power are cut, the particles trapped at positions floating above the wafer will tend to adhere to the wafer. There is therefore a demand for a method of suppressing the generation of these particles as far as possible, especially since circuitry patterns will continue to become finer in the future.
The present invention was devised in the light of the above problems with the conventional art and has as an objective thereof the provision of a plasma processing method that is capable of suppressing the creation of particles, thus increasing the yield.